Monitoring the Impact and Recovery of the Biota of the Rondegat River after the Removal of Alien Fishes

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In the South Africa’s Cape Floristic Region, the impact of alien fishes is at its most severe.
Invaded river areas are without indigenous fishes, in general. Indigenous aquatic species only occur upstream, above a waterfall or another barrier.
South African fish conservation experts consider the eradication of the alien fish as one of the best and fastest ways of improving the conservation status of the highly threatened fishes and aquatic biota. The purpose of the research was to quantify the initial impact of an eradication treatment, and to examine the nature and extent of ecological recovery.
CapeNature developed a dedicated project embracing the dual concepts of river rehabilitation and alien fish eradication, following a comprehensive and exhaustive Environmental Impact Assessment (EIA) on four rivers selected for pilot studies. This EIA recommended that the first river to be rehabilitated should be the selected reach of Rondegat River in the Cederberg, Western Cape, where the lower reaches are dominated by smallmouth bass and bluegill sunfish.
Following positive outcomes of extensive stakeholder consultation about the EIA recommendations the treatment was initiated.
The specific objectives attempted by this project were:
• Determine the impact of Cape Nature’s treatment on the indigenous aquatic biota, specifically fish, amphibians and macro-invertebrates
• Create a baseline dataset for future recovery monitoring of the fish, amphibians and invertebrates of the Rondegat river
• Develop a protocol to guide future treatments, including Lessons Learned.
Surveys of fish and aquatic biota were conducted in May 2010 and again in February 2012, pre and post treatment in February 2012 and again in May 2012, two months after the rotenone treatment. To avoid killing many fishes unnecessarily, a rescue effort was undertaken prior to the rotenone treatment.
The key finding of the monitoring programme is that CapeNature’s river rehabilitation has been a success. All bass appear to have been removed from the treatment area without significant long-term damage being accrued by the other faunas of the Rondegat River. By examining the invertebrate community at the species level over several seasons, the team was able to place the effect of rotenone on local diversity in context, and found it to be a minor negative effect in relation to natural seasonal fluctuations in the presence and absence of individual taxa.